This invention relates generally to electrical propulsion systems for traction vehicles (such as diesel-electric locomotives) equipped with either direct current or alternating current traction motors, and it relates more particularly to improved means for protecting such a system from serious damage in the event of an overvoltage reflected onto a field winding of a synchronous generator as a result of a shoot-through condition in a power circuit coupled to an output of the generator.
In a modern diesel-electric locomotive, a thermal prime mover (typically a 16-cylinder turbocharged diesel engine) is used to drive an electrical transmission comprising a synchronous generator that supplies electric current to a plurality of electric traction motors whose rotors are drivingly coupled through speed-reducing gearing to the respective axle-wheel sets of the locomotive. The generator typically comprises a main 3-phase traction alternator, the rotor of which is mechanically coupled to the output shaft of the engine. When excitation current is supplied to field windings on the rotating rotor, alternating voltages are generated in the 3-phase armature windings on the stator of the alternator. These voltages are rectified and applied to the armature and/or field windings of the DC traction motors or inverted to AC and applied to AC traction motors.
In normal motoring operation, the propulsion system of a diesel-electric locomotive is so controlled as to establish a balanced steady-state condition wherein the engine-driven alternator produces, for each discrete position of a throttle handle, a substantially constant, optimum amount of electrical power for the traction motors. In practice, suitable means are provided for overriding normal operation of the propulsion controls and reducing engine load in response to certain abnormal conditions, such as loss of wheel adhesion or a load exceeding the power capability of the engine at whatever engine speed the throttle is commanding. This response, generally referred to as deration, reduces traction power, thereby helping the locomotive recover from such temporary conditions and/or preventing serious damage to the engine.
In addition, the propulsion control system conventionally includes means for limiting or reducing alternator output voltage as necessary to keep the magnitude of this voltage and the magnitude of load current from respectively exceeding predetermined safe maximum levels or limits. Current limit is effective when the locomotive is accelerating from rest. At low locomotive speeds, the traction motor rotors are rotating slowly, so their back EMF is low. A low alternator voltage can still produce rated motor flux and maximum motor current which in turn produces the high tractive effort associated with low speeds. On the other hand, the alternator voltage magnitude must be at a higher level whenever locomotive speed is high. At high speeds the traction motor rotors are rotating rapidly and have a high back EMF, and the alternator voltage must then be high to produce the required flux in the traction motors. The power system for AC motors can exhibit a condition, commonly referred to as "shoot-through", which can have detrimental effects on the synchronous generator or alternator. In a typical AC traction motor system, the power output of the traction alternator is supplied to a rectifier circuit which converts the AC output of the alternator to DC. This DC power is then inverted by a solid state inverter into a frequency controlled AC power for application to the AC motor. The speed of the AC motor is controlled by the frequency of the supplied AC power. The inverter is conventionally arranged to provide 3-phase AC power and includes a plurality of controllable rectifiers such as silicon controlled rectifiers (SCR) or gate turnoff (GTO) thyristors. Each phase has at least two such devices connected in series between the relatively positive and relatively negative DC power buses extending from the rectifier circuit. During motoring operation, one of the devices in a phase is always off while the other device is conducting. If both devices were conducting simultaneously, the devices would form a short circuit across the rectifier output buses. Such a condition is referred to as a shoot-through and can result in very high currents.
Various failures can contribute to a shoot-through condition. For example, one device may simply fail to commutate off before another device begins conducting. More commonly, one device initially fails to a short-circuit condition and the second device in series with it is gated into conduction resulting in a short circuit between the DC power buses. The deration function of the propulsion system cannot respond sufficiently fast to prevent damage to the power system.
U.S. Pat. Nos. 5,168,416 and 5,245,495 describe one form of protection circuit for a DC electric traction motor using a series connected solid state switching device to disconnect the alternator field winding from its power source upon detection of a high current surge. One disadvantage of this system is that the series switching device, e.g., a GTO, must be sized to carry alternator field current during normal system operation. Further, the series device requires forced air cooling to prevent overheating and its stress level is high due to the continuous current it must carry.
As discussed above, the 3-phase synchronous generator in a locomotive propulsion system develops an output voltage which is a function of its rotor shaft RPM and the DC current applied to its field windings. The 3-phase output is converted to DC power by a 3-phase full bridge rectifier connected to the generator armature windings. This rectifier may contain fuses which function as protective devices to protect the alternator from overvoltages caused by failure of a device in or connected to the rectifier. The devices are typically solid-state diodes and fail to a "short-circuit" condition. In an AC motor system, the DC power is applied to an inverter and inverted to a controlled frequency power. Short-circuit protection for the inverters has normally been provided by fuses such as those described above for the rectifier circuit. The fuses are often a maintenance problem, since they last only about four years in the most severe locomotive conditions (e.g., pulling coal up a steep grade, i.e., low speed, maximum power, and at the highest output rectifier currents). When a fuse blows, the locomotive has to operate either at a reduced horsepower or not at all. Accordingly, it is desirable to provide a protection system which does not rely solely on fuses for short-circuit protection and avoids the need for series connected power devices and associated control circuitry.